The invention relates to an insulated-gate field-effect transistor which is adapted for detecting and measuring various chemical properties such as ion activity in a solution.
The measurement and monitoring of chemical properties, such as the presence, concentration and activity of particular ions, enzymes, antibodies, antigens, hormones and gases, are important in medical diagnosis and treatment and many other fields. Several attempts to adapt an insulated-gate field-effect transistor (IGFET) to facilitate such measurements have been reported. Among these are the article by P. Bergveld, "Development, Operation and Application of the Ion-Sensitive Field-Effect Transistor as a Tool for Electrophysiology", IEEE Trans. on Bio-Med. Eng., Vol. BME-19, No. 5, pp. 342-351, September 1972; the article by T. Matsuo and K. D. Wise, "An Integrated Field-Effect Electrode for Biopotential Recording", IEEE Trans. on Bio-Med. Eng., pp. 485-487, November 1974; and U.S. Pat. No. 4,020,830, "Selective Chemical Sensitive FET Transducers", issued May 3, 1977 to C. C. Johnson et al.
A conventional IGFET comprises a semiconductor substrate, a source region and a drain region. The source region is spaced apart from the drain region and both are located at or near one surface of the substrate. The region of the substrate between the source and drain is called the channel. The gate insulator is a thin layer of insulating material which covers the surface of the channel. The gate electrode is a layer of metal which covers the gate insulator. When an electric potential is applied to the gate electrode, the electric field in the gate insulator is modified. The electric field attracts or repels charge carriers, electrons or holes, in the adjacent semiconductor material thereby changing the conductance of the channel. The change in conductance of the channel is related to the signal applied to the gate electrode and can be measured by a current meter connected in series with a potential source, the source region, and the drain region.
In the previously mentioned attempts to adapt IGFET's to chemical measurements, the conducting metal layer in contact with the gate insulator of a conventional IGFET was omitted or replaced by an ion sensitive membrane. When the gate insulator or the membrane was exposed to an ionic solution, an electric field was induced in the gate insulator. As in a conventional IGFET, this electric field was sufficient to alter the conductance of the channel between the source and the drain regions.
These prior devices had several disadvantages. First, the gate insulator was a thin layer of silicon dioxide which was in close proximity to the test solution, whether the insulator was directly exposed to the solution or covered with a thin membrane. Because of this proximity, the gate insulator was easily contaminated by the solution. Certain contaminants, such as sodium ions, have a very high mobility in silicon dioxide. Thus, the resistance and other critical properties of the gate insulator were likely to be altered by exposure of the device to a solution. As a result, the response of the device varied greatly with time and exposure.
Second, the electrical contacts to the source and the drain region were also in close proximity to the solution. Contamination of the contacts and the remainder of the device could be limited by a protective layer which was impervious to the solution. However, the sequence and process steps for making the contacts, membrane, and protective layer had to be chosen carefully in order to be compatible.
Finally, the active surface area of the device which could be exposed to the solution was quite small because it was limited by the size of the gate region, i.e., by the short distance between the source and drain regions which was typically 20 .mu.m.